The present invention relates generally to the manufacture of synthetic polymers, and more particularly, to an improved method of manufacturing polyvinyl chloride (PVC) via the suspension polymerization of vinyl chloride or mixtures of vinyl chloride and other monomers which are copolymerizable therewith.
In the manufacture of polyvinyl chloride through suspension polymerization techniques, vinyl chloride monomer or mixtures of vinyl chloride and other monomers copolymerizable therewith are combined with a suspending agent and various other known additives in a polymerization reactor under pressure. The reaction is then allowed to continue for a period of time until the desired degree of polymerization has occurred. It has been found that the suspending agent, various additives to the reaction and the various reaction variables including the temperature at which the reaction is carried out, can all affect not only the reaction itself but also the properties of the resulting polyvinyl chloride including uniformity of particle size, narrowness of particle size distribution and the ability to adsorb plasticizer.
For the last decade, nonionic cellulose derivatives such as methylhydroxypropyl cellulose (MHPC) and polyvinyl alcohol (PVA) products, along with various other dispersing agents have been used as suspending agents for vinyl chloride polymerization. Both of the above product types have disadvantages that prevent their use over the full temperature range of PVC production. Because cellulose derivatives are not soluble in hot water higher than about 60.degree.-65.degree. C., use of cellulosics are limited at high temperatures. Thus as the temperature of the polymerization reaction approaches these temperatures, precipitation of cellulose derivatives becomes an increasing problem. This is a particular disadvantage since many of the more rigid PVC's are made at temperatures of about 70.degree. C. and higher. The polyvinyl alcohols are soluble at the higher temperatures mentioned above, but by themselves do not give fully acceptable porosity properties. During the last couple of years, polyvinyl alcohol has been combined with a secondary suspending agent such as partially hydrolyzed polyvinyl acetate to improve porosity of the resulting PVC. These secondary suspending agents, however, are quite expensive, thus increasing the overall cost of the PVC production.
Accordingly, the manufacture of PVC by the suspension polymerization method requires at least two different suspending agents, depending upon the temperature at which the polymerization is carried out. This requires maintaining a supply of at least two different suspending agents and in some cases, requires different sizes or types of reactors, depending upon the suspending agent used. As a result, there is a need for an improved method of manufacturing PVC using the suspension polymerization method which can be carried out using a single suspending agent regardless of the temperature of the reaction.